1. Field of the Invention
The present invention is concerned with a greatly improved system for latent or invisible imaging of printed material onto a variety of substrates, with subsequent development of the latent image by application of a corresponding developing fluid. More particularly, it is concerned with such a latent imaging system, and a novel developer forming a part thereof, which makes use of a dissociable transition metal salt in the imaging ink, and a color precursor in the developer fluid which includes an oxalic acid derivative capable of complexing with the transition metal ion to give a sharp, long lasting, non-smearing color.
2. Description of the Prior Art
Invisible or sympathetic inks, or more commonly "secret writing", have been a staple of romantic liasons and espionage activities for many centuries. Indeed, reports of such "secret writing" date back to antiquity.
However, most modern day sympathetic ink systems of commercial importance are designed for less adventuresome environments of use, for example in self-testing educational booklets. In such uses, a student may be provided with a multiple choice answer sheet, with the correct answer being indicated by appropriate latent image indicia. In selecting an answer, the student marks one of the multiple choice possibilities with a developer substance; if his selection is correct, the latent indicia will immediately reveal the student's correct choice and hence his progress. Other uses include certification testing, personnel selection and screening tests, novelty items such as games and toys, children's books designed for educational and/or entertainment purposes, securing of documents, prize verification and promotional items.
Systems of the type described typically involve application of a latent image by means of a normally invisible printing ink. Thereafter, the developer substance is applied over the invisible image and a color-forming chemical reaction occurs to "develop" the image and render the same visible. For example, U.S. Pat. No. 3,632,364 describes a latent imaging system wherein the invisible ink includes a soluble copper salt such as copper sulfate, while the developer includes a soluble iodide. In practice, the invisible ink is printed onto conventional paper, and is invisible to the naked eye. When the developer is applied, the iodide is oxidized by the copper ion present in the printed image to release iodine; the iodine in turn reacts with the starch in the paper to form a color. Other patents describing various latent imaging systems include U.S. Pat. Nos. 3,823,022, 4,051,283, 3,788,863, 3,451,143, 3,363,338 and 3,349,408. Other references include British Pat. No. 415,535 and an article entitled "Tintas Simpaticas" by P. L. De Araujo Feio, Revista Da Sociedade Brasileira de Quimica, Boletin Cientifico, Vol XIV, Janeiro-Marco de 1945, N. 1, Rio de Janeiro, Brazil. While certain latent imaging systems of the type described have found commercial application in, e.g., education selftesting materials and the like, they are typically plagued by a number of deficiencies. For example, the copper sulfate/soluble iodide system tends to develop an unstable color which will disappear under prolonged heating or ultraviolet light conditions. Moreover, in many instances the visual image lacks sharpness and clarity, and the color can smear if repeated applications of the developing fluid are made. Furthermore, use of this type of system is in practice generally limited to paper substrates, because of the necessity of having starch present as a part of the overall reaction. Thus, such a system is much more difficult to employ in the case of non-paper substrates such as metal or synthetic resin bodies.